The US neighbourhood watch volunteer who shot dead an unarmed black teenager is now a free man, but the Justice Department said it is looking into Trayvon Martin's death to determine whether federal prosecutors will file criminal civil rights charges against George Zimmerman.

President Barack Obama on Sunday described the death of Trayvon as “a tragedy”, but as protests began across the US, the President called on Americans to accept the acquittal of the teenager’s killer.

In a statement issued by the White House, the President acknowledged that passions were running high in the wake of the not guilty verdict. “But we are a nation of laws, and a jury has spoken,” he said.

Protests broke out across the US after Mr Zimmerman was cleared of all charges against him.

Authorities had asked the residents of Sanford, Florida, where the killing and subsequent trial took place, to keep the peace after hearing the verdict on Saturday night in a case that has gripped the country.

As dawn broke the riots feared by the police had failed to materialise, but peaceful protests had been reported as far afield as Washington, New York, Chicago, Philadelphia, Atlanta and San Francisco. In Los Angeles, around 200 demonstrators gathered for a vigil in one of the city’s African-American neighbourhoods, Leimert Park. Police in Oakland, California, said a small crowd took to the streets, with some breaking windows, spraying anti-police graffiti and burning US and California state flags. No arrests were made.

Mr Zimmerman, then 28, killed 17-year-old Trayvon on the evening of 26 February 2012 at The Retreat, a gated community on the outskirts of Sanford. The jury of six women heard testimony from more than 50 witnesses during the five-week trial.

Mr Zimmerman had been charged with second-degree murder, though the jurors were also given the option of convicting him on the lesser charge of manslaughter. They chose neither. Judge Debra Nelson warned the crowded courtroom that there should be “no outbursts upon the reading of the verdict or afterwards”. Mr Zimmerman appeared emotionless as he was declared not guilty, but smiled as Judge Nelson informed him that he was free to go. The verdict implies that Mr Zimmerman acted in self-defence, and was justified under Florida law in using deadly force “to prevent imminent death or great bodily harm” to himself.

On the night in question, Mr Zimmerman, a resident of The Retreat, was driving through the neighbourhood when he spotted Trayvon in the rain wearing a grey hoodie. The teenager had just bought a soft drink and a packet of Skittles at a local convenience store, and was returning to the nearby house where he was staying with friends.

Mr Zimmerman called police, claiming Trayvon looked suspicious. The dispatcher told him to wait for officers to arrive. “Fucking punks,” said Mr Zimmerman. “They always get away.”

What happened next may never be clear. Prosecutors said Mr Zimmerman left his car to pursue Trayvon on foot. Mr Zimmerman’s lawyers claimed Trayvon confronted and attacked their client, knocking him to the ground, striking his head on the pavement and reaching for his gun. The altercation ended with Mr Zimmerman firing a shot from his 9mm pistol that hit Trayvon in the chest.

The police declined to charge Mr Zimmerman due to Florida’s “stand-your-ground” law, which allows anyone who considers themselves in danger to take whatever action they deem appropriate to defend themselves. Civil rights leaders argued that the half-Peruvian Mr Zimmerman targeted Trayvon – who had no criminal record – because he was black. A month after the shooting, President Obama addressed the issue, saying: “If I had a son, he’d look like Trayvon.”

Finally, on 11 April 2012, the Florida State Attorney Angela Corey charged George Zimmerman with murder. Jurors were asked to decide whether the defendant was a dangerous vigilante or a concerned citizen backed into a corner. They were not helped by the sharply contradictory testimony of witnesses: one, Jonathan Good, told the court he saw Trayvon on top of Mr Zimmerman as the two fought; another, Selma Mora, said Mr Zimmerman had the upper hand before the fatal shot rang out. Even the screams heard on a 911 call remained contentious: the mothers of both the victim and the defendant claimed the cries for help belonged to their son.

Prosecutors did their best to persuade the court Mr Zimmerman had taken the law into his own hands. Prosecutor Bernie de la Rionda insisted: “A teenager is dead through no fault of his own. He’s dead because another man made an assumption.” Yet the evidence did not prove sufficient to secure a conviction. Mr Zimmerman’s lawyer, Mark O’Mara, told the jury Trayvon’s death was “a tragedy, but you can’t allow sympathy to feed into it”.

Mr O’Mara said he was “ecstatic” about the verdict, but that Mr Zimmerman, who has been in hiding and dons a bulletproof vest before venturing outside, would remain “very cautious”.

“Today … I’m proud to be an American,” Mr Zimmerman’s brother wrote on Twitter following the acquittal. Robert Zimmerman later stirred anger, however, with an appearance on CNN in which he suggested Trayvon had been “looking to procure firearms [and] growing marijuana plants”.

Civil rights leaders were outraged by the verdict, which the Rev Al Sharpton described as “an atrocity.” Roslyn M Brock, the chairman of the National Association for the Advancement of Coloured People, called on the US Department of Justice to investigate alleged civil rights violations committed against Trayvon.

Q&A: Was it really self-defence?

Q: What were the key points of the case against George Zimmerman?

A: That Zimmerman shot and killed Trayvon Martin was never in question. What motivated him to do so was. On the night of 26 February last year, Trayvon, 17, was walking through a gated community in Sanford, Florida, when he was spotted by Zimmerman, the neighbourhood watch volunteer. Zimmerman called the police on a non-emergency line, telling them he had spotted someone on the estate and telling the dispatcher: “Fucking punks. These assholes, they always get away.”

Q: Was Trayvon’s race important?

A: This gets to the heart of the controversy. Zimmerman’s opponents believe he followed Trayvon only because he was a young black man wearing a hooded top in a predominately white neighbourhood. Zimmerman was ruled to have shot Trayvon in self-defence – cuts and bruises on the back of his head were evidence that the pair had fought, and that was enough for the jury to clear him of second-degree murder.

Q: So is self-defence a plausible explanation?

A: On recordings of a 911 call made by one of George Zimmerman’s neighbours as the pair were fighting, a voice can be heard pleading for help. The prosecution claimed this was Trayvon, begging Zimmerman not to shoot him. The defence said the voice on the tape was in fact their client’s, that it was Zimmerman in trouble as Trayvon beat him. Neither side was able to prove beyond reasonable doubt who was speaking.

Q: What about the witnesses?

A: The prosecution’s case hinged on testimony from Trayvon’s friend, Rachel Jeantel, who was on the phone with him not long before he died. She said Trayvon kept complaining that a man was watching him, and that she heard Trayvon tell someone to “get off” before the line went dead. Ms Jeantel, 18, lied about her age in when questioned by police, telling detectives that she was a minor because she “didn’t want to get involved”. This, along with her evident annoyance under cross-examination, appears to have been enough to discredit her, although the prosecution again say attitudes towards her were influenced by the colour of her skin.

The defence called an expert witness who claimed the angle of the shot that killed Trayvon proved that he was on top of Zimmerman, arguing that the hole in his sweatshirt indicated that it was hanging away from his body, consistent with someone who was above the gun.

Q: And the police?

A: The police came in for heavy criticism throughout the trial. Sanford police chief Bill Lee allowed Zimmerman to walk free after he shot Trayvon, insisting that there was no evidence to suggest that he had not been acting in self-defence, and invoking the state’s “stand your ground” law, which permits those in fear for their lives to use deadly force. Zimmerman was eventually arrested six weeks after the shooting, and Lee was subsequently removed from his post and replaced by Cecil Smith, the first African-American to take charge of law enforcement in the area. Police are now attempting to prevent unrest in the wake of the verdict.